Juvenile banded catfish(Pseudobagurus fulvidraco, mean length $10.7{\pm}0.42cm$ and mean weight $15.0{\pm}0.23g$) were exposed to varies TAN(total ammonia) concentrations at pH levels of $6.12{\pm}0.51$, $7.00{\pm}0.26$ and $8.04{\pm}0.07$ for 96hrs to check the level of acute toxicity on biofloc technology aquaculture system(BFT). The result showed that cumulative mortalities for juvenile banded catfish at TAN levels 48.95, 55.96, 66.47, and 78.88 mg/L at pH $6.12{\pm}0.51$ were 0, 30, 30, and 50%, respectively. At pH $7.00{\pm}0.26$, its mortalities to TAN 5.20, 11.68, 15.31, and 18.31 mg/L were 0, 10, 20, and 70%, respectively and at pH $8.04{\pm}0.07$, the mortalities to TAN 0.96, 1.49, 2.13, and 3.62 mg/L were 10, 20, 40, and 100%, respectively. Its $96h-LC_{50}$ (median lethal concentration, $LC_{50}$) at pH $6.12{\pm}0.51$, $7.00{\pm}0.26$, and $8.04{\pm}0.07$ were 78.12, 15.87, and 2.21 mg/L for TAN, and 0.05, 0.10, and 0.14 mg/L for $NH_3$, respectively, and the acute toxicity for ammonia to juvenile banded catfish increased exponentially with increase of pH.